Clean Architecture

111
14
C o m pon e nt 
C ou plin g
The next three principles deal with the relationships between components. 
Here again we will run into the tension between develop-ability and logical 
design. The forces that impinge upon the architecture of a component 
structure are technical, political, and volatile.
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Chapter 14 Component Coupling
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Th e Ac yc l i c D e pe n d e n c i e s Pr i n c i pl e
Allow no cycles in the component dependency graph.
Have you ever worked all day, gotten some stuff working, and then gone 
home, only to arrive the next morning to find that your stuff no longer 
works? Why doesn’t it work? Because somebody stayed later than you and 
changed something you depend on! I call this “the morning after syndrome.”
The “morning after syndrome” occurs in development environments where 
many developers are modifying the same source files. In relatively small 
projects with just a few developers, it isn’t too big a problem. But as the size 
of the project and the development team grow, the mornings after can get 
pretty nightmarish. It is not uncommon for weeks to go by without the team 
being able to build a stable version of the project. Instead, everyone keeps on 
changing and changing their code trying to make it work with the last 
changes that someone else made.
Over the last several decades, two solutions to this problem have evolved, 
both of which came from the telecommunications industry. The first is “the 
weekly build,” and the second is the Acyclic Dependencies Principle (ADP).
Th e We e k ly B u i l d
The weekly build used to be common in medium-sized projects. It works like 
this: All the developers ignore each other for the first four days of the week. 
They all work on private copies of the code, and don’t worry about 
integrating their work on a collective basis. Then, on Friday, they integrate all 
their changes and build the system.
This approach has the wonderful advantage of allowing the developers to live 
in an isolated world for four days out of five. The disadvantage, of course, is 
the large integration penalty that is paid on Friday.
Unfortunately, as the project grows, it becomes less feasible to finish 
integrating the project on Friday. The integration burden grows until it starts 
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